With a pulse radar sensor, distances of objects can be determined very exactly. Therefore, a pulse-radar-based device in vehicles is suitable for instance as a parking aid or as a precrash sensor, or for detecting objects in the region of what is known as the blind angle. In all these usage instances, the goal is to monitor the near surroundings of a vehicle for whether obstacles that could be collided with are located in that region and to find the distance between these obstacles and the vehicle. Pulse radar sensors have an oscillator in the gigahertz range. In the course of mass-production development, it is desirable to be able to perform frequency and power calibrations for such oscillators in an automated way.
It is known to perform the power calibration for an oscillator via a buffer amplifier that directly follows the oscillator. The amplifier is connected to the oscillator output via an HF switch, which as a rule has two diodes. This buffer makes it possible to vary the HF output power by changing the drain voltage of the amplifier, which may be implemented as a simple transistor. After a power determination at the output of the buffer, it is possible with the aid of the control module (such as a closed control loop) to readjust the drain voltage, and thus over the course of temperature and process and component fluctuations, a constant output power can be attained. An upper limit to this output power is set by certification regulations, and this upper limit should always be fully exploited if maximum power is to be obtained. The measured HF output power serves in this case as a controlled variable, for readjusting the drain voltage of the “buffer” final control element. In the prior art, it is known for a detector diode, which generates a direct voltage proportional to the power, to be used as a measuring member. This detector diode is coupled to the line, and the regulation is designed in accordance with the characteristic curve of the diode, including the coupling.
A disadvantage of the known devices in the prior art is particularly the use of an expensive semiconductor (the detector diode), as well as the not inconsiderable space required for this component.